1. Field of the Invention
This invention relates in general to prosthetic devices, and more particularly to articulated prosthetic ankle joints.
2. Description of the Related Art
Currently available prosthetic ankle joints are heavy, bulky, and do not provide a range of motion that imitates a normal foot range of motion during walking. Each foot and ankle during walking travel through stance and swing phases of a gate cycle. In the stance phase, the foot is in contact with the ground and the weight of a person is supported on the foot. In the swing phase, the foot is off the ground as the entire leg and foot move from a posterior position to an anterior position with respect to a center of gravity of the person. The stance phase begins just after completion of the swing phase and commences with a heel strike wherein the foot is lowered to the ground as the body moves forward from a position posterior to the person's center of gravity. Immediately after heel strike, the foot moves from a dorsiflexed position, wherein the toes of the foot are pointed upwards, to a plantarflexed position wherein the bottom of the foot or shoe is flat on the walking surface, which provides greater stability as the entire weight of the person is shifted over the foot in contact with the ground. The swing phase commences just after heel strike of the other foot. During the swing phase, the foot is again in the dorsiflexed position as the foot leaves the walking surface and the foot and leg swing forward in preparation for the stance phase. Dorsiflexion is very important for normal human locomotion, since the toes must be dorsiflexed in order to clear the floor. If the foot were not dorsiflexed during the swing phase, it would most likely catch on the walking surface and cause the person to stumble and fall, leading to potentially serious injury.
With an amputee, it is customary to make the prosthetic limb about 3/8 to 1/2 inch shorter than the natural limb in order to provide adequate ground clearance for the prosthetic limb during the swing phase. Consequently, the amputee develops an unnatural gate pattern, causing him or her to lean to one side while walking. Over the years, this unnatural gate pattern may cause hip, pelvis, knee and back problems.
In addition, most prosthetic feet and ankle systems are fixed at 90.degree.. During heel strike of the prosthetic foot, the momentum of the person causes the foot to rotate forward to a more stable flat position on the walking surface. However, since there is no flexibility in the ankle, the person is lurched forward. As the person's center of gravity passes over the centerline of the foot, the knee is forced to bend to compensate for the unnatural movement. With above-knee amputees, the prosthetic knee joint may become unstable and cause unwanted or unexpected knee flexure, and may cause the person to fall.